Patch gone mainstream. No need apply patch to the extension

TagSidebar Offical repository

TagsSideBar is a firefox add-on allowing you to search in your bookmarks for specifics tags. Badly, it doesn't work with newer versions of Firefox.

The idea sounds great. Remember, Firefox uses SQLite for storing places, it has a complete API for querying the places database but nothing for using it. After a few modifications, I was able to run TagsSideBar with newer versions of Firefox.

Sidebar.js sort-of-patch (xpi is a zip file)

        this.getBookmarksForSelectedTags = function() {
            let queryOptions = this.historySrv.getNewQueryOptions();
            let query = this.historySrv.getNewQuery();
    -       query.tags = this.selectedTags;
    +       let bmfolder = this.bookmarksSrv.bookmarksMenuFolder;
    +       let ubfolder = this.bookmarksSrv.unfiledBookmarksFolder;
    +       let tbfolder = this.bookmarksSrv.toolbarFolder;
    +       query.searchTerms = this.selectedTags.sort();
    +       query.setFolders([ubfolder,bmfolder,tbfolder], 3);
            return this.historySrv.executeQuery(query, queryOptions);
        }; // end TagsSidebar.getBookmarksForSelectedTags()

And for tags sort.

        let query = this.historySrv.getNewQuery();
        let tagsFolder = this.bookmarksSrv.tagsFolder;
        query.setFolders([tagsFolder], 1);
+       options.sortingMode = options.SORT_BY_KEYWORD_ASCENDING;
        return this.historySrv.executeQuery(query, options);
    }; // end TagsSidebar.getTags()

You can download the patched version of TagSideBar here.